This invention relates in general to stripline filters. More particularly, the invention relates to the structure of the resonators employed in such stripline filters.
Those skilled in the art use the term "stripline" to refer to structures which include a layer of dielectric material having opposed surfaces on which respective layers of electrically conductive material are disposed. One or more resonators are sandwiched within the dielectric layer to fabricate as stripline filter structure.
For example, FIG. 1 shows one such conventional sandwiched stripline resonator structure as stripline structure 10 prior to completion of fabrication. Stripline structure 10 includes layers 20 and 30 of dielectric material such as Teflon.RTM. material. Layer 20 includes opposed major surfaces 20A and 20B. Layer 30 includes opposed major surfaces 30A and 30B. Ground plane layers 40 and 50 of electrically conductive material are situated on surfaces 20A and 30B, respectively, as shown. A perspective view of stripline structure 10 is shown in FIG. 2 to more clearly illustrate the components thereof. A resonator 60 is disposed on surface 30A as shown in FIG. 2. Resonator 60 is a substantially rectangular strip of electrically conductive material having a length corresponding to the frequency at which resonator 60 is desired to resonate.
To complete fabrication of stripline structure 10, dielectric layers 20 and 30 are situated adjacent each other as shown in FIG. 3. The combined stripline structure is then heated such that Teflon.RTM. dielectric layers 20 and 30 become plastic and encapsulate resonator 60 therebetween.
Although the stripline resonator structure of FIG. 3 performs acceptably as a resonator, it exhibits current bunching at the cross-sectional corners thereof. FIG. 4 shows a simplified representation of current density at different points around the periphery of the cross-section of the rectangular resonator 60. Significant current bunching is observed at resonator corners 60A, 60B, 60C and 60D. This nonuniform current density or current bunching effectively increases the alternating current resistance exhibited by resonator 60. It is well known that such increases in resonator resistance correspondingly degrade the quality factor or Q of the resonator.
For purposes of this document, Q.sub.U is defined as the unloaded quality factor of a particular resonator which is uncoupled to any adjacent resonators. Q.sub.L is defined as the loaded quality factor of a particular resonator which is coupled to a resistive source or load. The ratio Q.sub.L /Q.sub.U of adjacent resonators determines the passband insertion loss of a stripline filter which employs such resonators. That is, the lower the loaded Q or Q.sub.L, for a given Q.sub.U, then the lower is the insertion loss of the stripline resonator filter. Resonators with a low Q.sub.L /Q.sub.U ratio result in filters with low insertion loss. Thus, resonators in which nonuniform current distribution results in high effective resistance also results in low unloaded Q and high insertion loss.
FIG. 5 shows another resonator structure 100 substantially similar to the resonator structure 10 of FIG. 1-3 except for the following modifications. In resonator structure 100, dielectric layers 20 and 30 are fabricated from ceramic material instead of a dielectric material which becomes plastic when heated as in resonator structure 10. Moreover, in resonator structure 100, a resonator portion 70 is situated on surface 20B in the same manner that resonator portion 60 is situated on surface 30A. FIG. 5 shows dielectric layers 20 and 30 prior to being sandwiched together to form a stripline resonator. Resonator portions 60 and 70 are aligned and soldered together as shown in FIG. 6 to form a resonator 80. When dielectric layers 20 and 30 are sandwiched together to form the resonator structure of FIG. 6, it is noted that resonator 80 separates surfaces 20B and 30A. In so doing, resonator 80 forms an air gap 110 between dielectric layers 20 and 30. Although resonator structure 100 of FIG. 6 generally performs well, unfortunately the air gap 110 which is characteristic of this structure is subject to contamination which can lead to performance degradation.